A common complication of fracture repair, external fixation (or other orthopedic procedures in which fasteners are inserted into bone) is loosening of the fastener over time at the fastener/bone interface. In their article “The Effect of Radial Preload on the Implant/Bone Interface: A Cadaveric Study”, Biliouris et al. suggest several reasons for this loosening, including micro-movement of the bone tissue as axial or bending loads are exerted on the fastener during normal use of the fastened bone by the patient after surgery. As described in some detail in the above-referenced article (hereafter referred to as “Biliouris et al.”), such axial and bending loads can cause circumferential and other micro-displacement of the fastener as secured into the bone. This micro-displacement temporarily deforms the shape and size of the bone hole into which the fastener is secured, resulting in loss of circumferential contact between the fastener and the bone, which in turn leads to higher stress conditions at the remaining areas of contact. However, while the higher elasticity of the metal fastener allows the fastener to resume its original shape and relative position within the bone hole after loading, the lower elasticity of the bone surrounding the bone hole leaves the hole potentially damaged after loading. As a result, there is a loss of contact between fastener and the bone during and potentially after loading, resulting in micro-motion between fastener and bone, manifesting itself over time and repetitive loading as loosening of the fastener.
Biliouris et al. also discuss the use of radial preload to counteract loosening of fasteners under repetitive loads in external fixation. The basic concept of radial preload is to “oversize” the diameter of the fastener within the bone hole. In this way, the bone material around the “oversized” portion of the fastener is compressed, tightening the contact between the fastener and the bone material, encouraging intimate and sustained contact during operational loading, and thereby reducing the propensity for loosening.
However, as taught by Biliouris et al., the bone material's comparatively low elasticity limits the amount of radial preload that can be absorbed by the bone material without causing cracking of the bone material around the oversized portion of the fastener. Biliouris et al. observe that a fastener diameter more than about 0.2 mm greater than the receiving bone hole is prone to cause micro-cracking in surrounding cortical bone material.
The reference in this disclosure so far to “bone” deserves further discussion. Orthopedic fasteners are typically designed to fasten either cortical bone or cancellous bone. Cortical bone, typically found on the outside of a bone, is much tougher and harder than cancellous bone, which is typically found on the inside of a bone. Cancellous bone has soft and malleable characteristics, whereas cortical bone is considerably harder. While radial preload is understood to provide advantages in both cortical and cancellous bone, the inventive focus of this disclosure is on the radial preload imparted by fasteners secured in cortical bone.
U.S. Pat. No. 6,949,100 (Venturini) discloses an orthopedic fixation pin with a tapered thread. While Venturini mentions use in the art of radial preload, Venturini's disclosure and invention focuses on thread profile geometry as a way to enhance the fixation pin's grip on bone.
U.S. Pat. No. 5,961,524 (Crombie) discloses an orthopedic fastener with a tapered thread. The fastener is configured to be received into a smooth hole of substantially the same taper as the thread on the fastener. Once secure into the into the hole, the fastener is given a small amount of extra tightening to compress bone material surrounding the threads in order to improve grip of the fastener on the bone. Crombie's disclosure, however, appears to be solving the problem of improving bone-to-fastener contact rather than imparting limited or controlled amounts of radial preload into the bone surrounding the fastener. As a result, no structure to measure or limit radial preload is disclosed.
U.S. Pat. No. 7,198,488 (Lang et al.), U.S. Pat. No. 5,593,410 (Vrespa) and U.S. Pat. No. 6,953,463 (West, Jr.) illustrate fastener thread styles understood to be advantageous in improving fastener grip in either cortical or cancellous bone. These disclosures provide no guidance, however, on imparting limited or controlled amounts of radial preload into the bone surrounding the fastener.
Likewise U.S. Pat. No. 6.565,573 (Ferrante et al.) and U.S. Pat. No. 6,375,657 (Doubler et al.) disclose bone fasteners with generally tapered threads, but the disclosures of these patents provide no guidance on imparting limited or controlled amounts of radial preload into the bone surrounding the fastener.
U.S. Pat. No. 7,001,389 (Navarro et al.) discloses fasteners with various thread configurations, including tapered threads, to assist in securing a plate to bone. As with other prior art discussed above, however, Navarro et al. provide no guidance on imparting limited or controlled amounts of radial preload into the bone surrounding the fastener.
There is therefore a need in the art for an orthopedic fastener specially designed to exert radial preload on the surrounding bone so that the radial preload reduces the likelihood of loosening of the fastener under repetitive loads. The diameter of such a new fastener will advantageously not exceed the diameter of the bone hole by more than about 0.2 mm along the portions where radial preload is being imparted. In this way, the amount of radial preload imparted into the bone will be limited to no more than about 0.2 mm. Such a new fastener may further, or alternatively, advantageously provide structure, in other embodiments, to engage a predetermined amount of radial preload for a particular installation.